The invention relates to an arrangement for recording or reproducing pictures comprising a cathode-ray tube having in an evacuated envelope a target plate and a cathode. This arrangement further includes means for forming a positive electron lens with the cathode, a first grid and a screen grid provided with an opening allowing the passage of electrons emitted by the cathode.
In an arrangement for recording pictures, the cathode-ray tube is a camera tube and the target plate is a photosensitive elements, such as a photoconducting layer. In an arrangement for reproducing pictures, the cathode-ray tube may be a picture tube, while the target plate comprises a layer or a pattern of lines or dots of a fluorescent material. Such an arrangement may also be designed for electron-lithographic or electron-microscopic applications.
Netherlands Patent Application No. 7905470 laid open to public inspection, and corresponding to U.S. Pat. No. 4,303,930, discloses a cathode-ray tube provided with a so-called "cold cathode". The operation of this cathode is based on the emanation of electrons from a semiconductor body in which a pn junction is operated in the reverse direction in such a manner that avalanche multiplication of charge carriers occurs. In this case, certain electrons can receive such an amount of kinetic energy as is required for exceeding the electron work function. These electrons are then released at the major surface of the semiconductor body and thus supply an electron current.
Since residual gases always remain in the evacuated envelope, negative and positive ions are released by the electron current from these residual gases. The negative ions are accelerated towards the target plate. In the case of electrostatic deflection, they can strike a small region of the target plate and damage the latter or adversely affect its operation. In order to prevent this harmful effect, ion traps are used. An ion trap for negative ions is known, for example, from U.S. Pat. No. 2,913,612.
Some of the positive ions move, under the influence of accelerating and focusing fields prevailing in the tube toward the cathode. If no special steps are taken, a number of these positive ions will strike the semiconductor and damage it. This damage may be a gradual removal by sputtering of a layer of material reducing the electron work function, such as, for example, cesium, which may be present. When this material is redistributed or even disappears completely, the emission properties of the cathode are changed. If this layer is not present (or is removed completely by the aforementioned sputtering mechanism), even the major surface of the semiconductor body may be attacked. In the case of a semiconductor cathode which utilizes avalanche multiplication of charge carriers such as the cathode described in Netherlands Patent Application No. 7905470, in which the emitting pn junction extends parallel to the major surface and is separated therefrom by an n-type surface zone, it is possible that due to this gradual sputtering this surface zone disappears completely so that the cathode is no longer operative. In a similar type of cold cathode as described in Netherlands Patent Application No. 7800987 in the name of the Applicant laid open to public inspection on 31 July 1979, and corresponding to U.S. Pat. No. 4,259,678, the pn junction is exposed at the major surface of the semiconductor body. Due to the damaging effect described above of positive ions present in the electron tube, for example the area at which the pn junction is exposed at the major surface may change. This leads to an unstable emission behaviour.
In the second type of cathode-ray tube, in which a pn junction is operated in the forward direction in the semiconductor cathode (the socalled negative electron affinity cathode or NEA cathode), the emission behaviour is also influenced due to the fact that sputtering takes place again. Also in this case, the layer of material reducing the electron work function is first gradually removed by sputtering. Subsequently, the n-type surface zone of the cathode is attacked until the cathode is no longer operative. Similar problems apply to other semiconductor cathodes, such as, for example, the semiconductor cathodes described in British Patent Applications No. 813359 (corresponding to U.S. Pat. No. 4,516,146) and No. 8133502 (corresponding to U.S. Pat. No. 4,506,284).
It is found that due to the aforementioned processes, the life of cathode-ray tubes manufactured with such semiconductor cathodes is considerably shortened.